Purpose
To perform a titration experiment with a strong base/ strong acid solution using 10.0 mL of 1.5M HCL to determine the concentration of the NaOH solution.
Procedure
1. Obtain 30mL of HCl with a concentration of 1.5 molarity.
2. Measure out 10mL of HCl using a graduated cylinder and pour it in a flask.
3. Put two drops of phenolphthalein into the flask. The solution should still be a clear color.
4. Place the flask under the buret. Take note of the initial amount of NaOH on the buret.
5. Slowly twist the knob to let the NaOH drop at a moderate rate into the flask. As the NaOH is dropping, the solution will turn pink but it goes away. Make sure to swirl the flask as soon as a drop of NaOH drops.
6. Start to slow down the dropping pace massively when the pink starts to linger a little longer. That means it’s about to reach equilibrium. You want to achieve a faint pink. A hot pink means you have gone a little over.
7. Once a faint pink (or hot pink) is achieved, retrieve ending amount of the NaOH left. Take the ending amount and subtract it from the initial and that’s the amount of mL used.
8. Dispose the solution in the drain and rinse the flask.
9. Repeat steps 2-8 three times.
2. Measure out 10mL of HCl using a graduated cylinder and pour it in a flask.
3. Put two drops of phenolphthalein into the flask. The solution should still be a clear color.
4. Place the flask under the buret. Take note of the initial amount of NaOH on the buret.
5. Slowly twist the knob to let the NaOH drop at a moderate rate into the flask. As the NaOH is dropping, the solution will turn pink but it goes away. Make sure to swirl the flask as soon as a drop of NaOH drops.
6. Start to slow down the dropping pace massively when the pink starts to linger a little longer. That means it’s about to reach equilibrium. You want to achieve a faint pink. A hot pink means you have gone a little over.
7. Once a faint pink (or hot pink) is achieved, retrieve ending amount of the NaOH left. Take the ending amount and subtract it from the initial and that’s the amount of mL used.
8. Dispose the solution in the drain and rinse the flask.
9. Repeat steps 2-8 three times.
Data Table
Calculations
Graph of pH vs. time for the above calculations
Conclusion
The purpose of this lab was to determine the concentration of the NaOH solution. This was made possible using the dilution titration formula M1V1=M2V2. The molarity and volume of HCl was given and the average volume of NaOH was used for V2. With that set up, the molarity of NaOH was figured out: 1.3M. Using that molarity, we were able to calculate changes in pH if the volume of NaOH increased.
Discussion of Theory
Titration is a procedure used to determine the concentration of a solution with an unknown concentration. Within the titration, there is an equivalence point which is when the reaction between the titrant and unknown has just been completed (there’s none left). The endpoint is when the indicator changes color. Color changes usually occur when the equivalence point is reached, but the difference between endpoint and equivalence point is one deals with the reaction and the other deals with color change specifically. An indicator is the substance that undergoes a color change. It is important to pick an indicator that changes around the equivalence point of the titration. Otherwise, the titration wouldn’t work properly.
For a strong acid-strong base titration, the concentration of H+ can be calculated before the equivalence point. However, in order to get to [H+], a series of steps must be done. First, list all the major species. This is key to setting up the BdeltaA (similar to ICE box) problem. Strong acid-strong base will have H+ and OH- ions, and those are the most significant ones that will affect pH. Set up the reaction as H+ + OH- --> H2O. Cross out water so you aren’t tempted to put something there because remember liquids and solids are not used. Next, use M1V1=M2V2 to get the mmol of H+ and OH-. Take the lowest value (usually OH- before E. Point is reached) and subtract it from itself and the other (H+). In order to find out the concentration of what you have left (H+ if before E. Point), take the value of mmol left and divide it by the new total volume. Then, to find the pH, do -log[H+]. At the equivalence point, the pH should be 7.00. After the equivalence point, [OH-] can be calculated from the excess OH- and the new total volume. Using [OH-], do -log[OH-] = pOH. pH + pOH = 14, so take pOH and subtract it from 14 and there’s the pH.
It’s basically the same steps for weak acid-strong base and weak base-strong acid. However, the reaction of OH- with the weak acid is assumed to run to completion. There would be the concentrations of the acid remaining and its conjugate base. Use the Henderson-Hasselbalch equation to determine the pH of this buffer. The pH at the equivalence point of a weak acid- strong base is always greater than 7. The pH of a weak base-strong acid is always less than 7.
A buffered solution resists a change in pH when either hydroxide ions or protons are added. Buffer solutions contain either a weak acid and its salt or a weak base and its salt. Buffered solutions contain large concentrations of a weak acid and the corresponding weak base. They can involve a weak acid and the conjugate base or a weak base and the conjugate acid. When H+ is added to a buffered solution, it reacts to completion with the weak base present. When OH- is added to a buffered solution, it reacts to completion with the weak acid present. The buffering capacity is the amount of protons or hydroxide ions the buffer can absorb without a significant change in pH. Of course, the buffering capacity is not perfect. A certain capacity will work, but it will eventually break down.
For a strong acid-strong base titration, the concentration of H+ can be calculated before the equivalence point. However, in order to get to [H+], a series of steps must be done. First, list all the major species. This is key to setting up the BdeltaA (similar to ICE box) problem. Strong acid-strong base will have H+ and OH- ions, and those are the most significant ones that will affect pH. Set up the reaction as H+ + OH- --> H2O. Cross out water so you aren’t tempted to put something there because remember liquids and solids are not used. Next, use M1V1=M2V2 to get the mmol of H+ and OH-. Take the lowest value (usually OH- before E. Point is reached) and subtract it from itself and the other (H+). In order to find out the concentration of what you have left (H+ if before E. Point), take the value of mmol left and divide it by the new total volume. Then, to find the pH, do -log[H+]. At the equivalence point, the pH should be 7.00. After the equivalence point, [OH-] can be calculated from the excess OH- and the new total volume. Using [OH-], do -log[OH-] = pOH. pH + pOH = 14, so take pOH and subtract it from 14 and there’s the pH.
It’s basically the same steps for weak acid-strong base and weak base-strong acid. However, the reaction of OH- with the weak acid is assumed to run to completion. There would be the concentrations of the acid remaining and its conjugate base. Use the Henderson-Hasselbalch equation to determine the pH of this buffer. The pH at the equivalence point of a weak acid- strong base is always greater than 7. The pH of a weak base-strong acid is always less than 7.
A buffered solution resists a change in pH when either hydroxide ions or protons are added. Buffer solutions contain either a weak acid and its salt or a weak base and its salt. Buffered solutions contain large concentrations of a weak acid and the corresponding weak base. They can involve a weak acid and the conjugate base or a weak base and the conjugate acid. When H+ is added to a buffered solution, it reacts to completion with the weak base present. When OH- is added to a buffered solution, it reacts to completion with the weak acid present. The buffering capacity is the amount of protons or hydroxide ions the buffer can absorb without a significant change in pH. Of course, the buffering capacity is not perfect. A certain capacity will work, but it will eventually break down.
Answers to Questions
1. The purpose of doing a titration is to neutralize a solution, but in this lab the purpose of the titration was to find the molarity of the NaOH that was added.
2. An indicator is the substance that undergoes a color change in the pH interval of the equivalence point. The color change of the indicator occurs when a proton is lost or gained. It is also dependant on when the concentration of the more dominant form is ten times as great as the less dominant form.
3. To decide which indicator should be used for a titration, choose one on the basis of pH a the equilvalence point of the two reagents. Otherwise, they will not be helpful in a titration. You need to know the approximate pH at the equivalence point. Use the indicator which changes around the equivalence point of the titration.
4. The difference between the equivalence point and the end point is that the endpoint is when the indicator changes color. The equivalence point is the point when the reaction between the titrant and the unknown has just been completed. In simple terms, end point has to do with color change and equivalence point has to do with the reaction.
2. An indicator is the substance that undergoes a color change in the pH interval of the equivalence point. The color change of the indicator occurs when a proton is lost or gained. It is also dependant on when the concentration of the more dominant form is ten times as great as the less dominant form.
3. To decide which indicator should be used for a titration, choose one on the basis of pH a the equilvalence point of the two reagents. Otherwise, they will not be helpful in a titration. You need to know the approximate pH at the equivalence point. Use the indicator which changes around the equivalence point of the titration.
4. The difference between the equivalence point and the end point is that the endpoint is when the indicator changes color. The equivalence point is the point when the reaction between the titrant and the unknown has just been completed. In simple terms, end point has to do with color change and equivalence point has to do with the reaction.
7. A buffer is either a weak acid and its salt or a weak base and its salt. A buffered solution is a solution that resists a change in pH when either hydroxide ions or protons are added. When H+ and OH- ions are added to a weak acid (which is the best source for more H+ ions or OH- ions), it forms H2O. The remaining conjugate reacts with H+ or OH-. Since Henderson-Hasselbalch’s equation is pH=pKa + log ([A-]/[HA]) (in which [A-] is base and [HA] is acid), if the concentration of the compound and its conjugate are large compared to the concentration of H+ or OH-, there would be very little change in pH.
8. a. H b. L c. N d. N e. H f. L g. H
8. a. H b. L c. N d. N e. H f. L g. H